Field of the Invention
The present invention relates to apparatus and method for supplying dilute gases at predetermined concentrations, e.g., as source gas for ion implantation doping of semiconductor or other microelectronic device materials.
Description of the Related Art
The semiconductor industry uses a wide variety of dilute gases in applications where the source material is highly toxic or hazardous and the dosage of active gas species is small.
For instance, ion implantation doping of epitaxial films by requires source gases such as arsine, phosphine, and germane in highly dilute states. As an example, arsenic may be implanted in a semiconductor film for doping thereof, from a dilute arsine/hydrogen gas mixture. In such arsenic doping application, a source gas of low arsine content e.g., 50 parts per million (ppm) may be further diluted with hydrogen to achieve a desired hydrogen/arsine gas mixture. The flows of the dilute arsine starting material and the diluent hydrogen that is added thereto to form the final dilute arsine gas mixture can be controlled by mass flow controllers, to deliver a metered amount of the final diluted arsine to the ionizer unit of the ion implant system.
Generally, two primary approaches are utilized in the semiconductor industry for supplying an active gas (such term being used hereinafter to designate the gas component of interest, such as the dopant gas species) in diluted form in a gas mixture useful for a desired application.
A first category of dilute gas supply techniques utilizes pre-mixed high-pressure gas mixtures (containing the low-concentration active gas component) as the source gas medium, as dispensed for use from high-pressure gas supply vessels such as pressurized gas cylinders. This gas supply approach has the following deficiencies:                (1) the gas supply vessels are exhausted at a high rate, requiring numerous change-outs of the gas supply vessels during the operation of the active gas-consuming process;        (2) when gas supply vessels are changed out as they are exhausted, the active gas-consuming process may need to be re-qualified, since the concentration of active gas supplied from a freshly installed gas supply vessel may be different from the concentration dispensed from a previously installed gas supply vessel;        (3) in addition to deficiency (2), the gas concentration of the active gas dispensed from any given gas supply vessel is fixed by the gas supply vessel manufacturer, and there is no capability of delivering varying concentrations depending on time-varying conditions in the downstream active gas-consuming process;        (4) the concentration of the active gas in the gas mixture stored in the gas supply vessel can change with time due to decomposition of the active gas component, or the concentration of the active gas can vary with successive change-outs of gas supply vessels, in an unknown and unexpected manner; and        (5) the gas supply vessel typically is at high atmospheric pressure to maximize inventory of the active gas in the vessel, entailing a potentially unsafe situation if the gas supply vessel ruptures or leakage from the associated head assembly, valves, etc. of the vessel occurs.        
The second general category of dilute gas supply techniques involves in-situ generation of gas, using solids or liquid raw materials to generate the desired gas species through chemical reaction. In-situ gas generation has the following associated deficiencies:                (1) the time required to initiate gas generation and achieve steady-state gas production is generally substantial and does not permit a quick-response turn-on of gas dispensing to be achieved;        (2) the raw materials used as reactants for in-situ gas generation are frequently highly toxic in character, thereby raising safety and operational issues;        (3) in-situ gas generators are typically relatively complex systems, including for example, gas generation chambers, reactant supplies, reactant flow circuitry (since even in the case of solid reactant sources, there is typically a fluid co-reactant), dispensing lines, and associated in-line filters, purifiers, interlocks, etc.;        (4) in-situ gas generators as conventionally employed involve consumable parts requiring periodic replacement, e.g., filters and purifiers; and        (5) in-situ gas generation systems are relatively expensive, both in capital expenditure and in overall cost of ownership.        
U.S. Pat. No. 7,063,097 issued Jun. 20, 2006 to Jose I. Arno and James A. Dietz for “In-Situ Gas Blending and Dilution System for Delivery of Dilute Gas at a Predetermined Concentration” describes an in-situ gas blending and dilution system for delivery of dilute gas at a predetermined concentration, which includes an active gas source and a diluent gas source. A gas flow-metering device is provided for dispensing the active gas at a predetermined flow rate. A gas blender mixer is arranged to mix (i) active gas from the active gas source that is dispensed at the predetermined flow rate by the gas flow-metering device, with (ii) diluent gas, to form a diluted active gas mixture. The system further includes a monitor arranged to sense concentration of active gas in the diluted active gas mixture and to responsively adjust the gas flow-metering device, to control the dispensing rate of the active gas, and maintain a predetermined concentration of active gas in the diluted active gas mixture.
In one embodiment of the system described in U.S. Pat. No. 7,063,097, as adapted for delivery of gas for ion implantation in a semiconductor manufacturing facility, the monitor includes a thermopile infrared (TPIR) detector, and the system utilizes a variable restricted flow orifice (RFO) as a flow control device for the source gas, and a mass flow controller (MFC) as a flow control device for the diluent gas, with a micro-pump to deliver a specific concentration of the source gas to the semiconductor manufacturing ion implant tool from the gas blender.
In such ion implantation system, it is desirable to be able to control the overall flow of the diluted gas mixture that is entering the ion implant tool from the gas blender. This is problematic, however, since any adjustment of the flow rate causes the pressure maintained inside the gas blender to change. This in turn disrupts the gas concentration signal that is being sensed by the TPIR detector, since the signal sensed by the TPIR detector is directly proportional to both temperature and pressure. Any change in pressure causes the calibration of the TPIR to be inaccurate, with the result that an incorrect concentration of the source gas in the diluted mixture is caused to be fed to the semiconductor manufacturing tool.
Additionally, in such ion implantation system, if the blender is operated at extremely low pressures, e.g., at less than 50 torr, the TPIR detector may be unable to sense any level of source gas.
This inability to accurately control concentration under adjusted flow rates, and potential inability to sense diluted source gas concentration at very low pressures, present significant operating issues.
The active gas source used in gas blender delivery systems of the type discussed above can include fluid storage and dispensing packages in which a physical adsorbent retains the active gas thereon in a vessel, for desorption of the active gas and discharge from the vessel under dispensing conditions. Such gas supply systems are commercially available from ATMI, Inc., Danbury, Conn., USA under the trademarks SDS and SAGE and are described, for example, in U.S. Pat. Nos. 5,518,528; 5,704,965; 5,704,967; and 5,707,424.
The active gas source used in the gas blender delivery system alternatively can include a fluid storage and dispensing package in which a pressure regulator is positioned in the interior volume of a vessel holding the active fluid under pressure. The pressure regulator is arranged with a set point permitting dispensing of gas deriving from the fluid, at pressure determined by the set point, e.g., a subatmospheric pressure providing a high level of safety in operation. Internal regulator gas supply packages of such type are commercially available from ATMI, Inc., Danbury, Conn., USA under the trademark VAC and are described, for example, in U.S. Pat. Nos. 6,101,816 and 6,089,027.
In the use of the above-mentioned SDS, SAGE and VAC packages for supply of active gas in the above-described gas blender delivery systems, it frequently is unclear to the user when the gas storage and dispensing package is approaching exhaustion. As a result of such uncertainty, the package may be taken out of service at a premature point in time, relative to the actual point of exhaustion, with consequent waste of residual active gas remaining in the package and adverse effect on the economics of the process using the active gas. Alternatively, the package may continue to be operated until total exhaustion of the active gas has occurred, and the package is “running dry.” As a result, the process being serviced by the empty package must be stopped, to accommodate change out of the gas supply package and introduction of a fresh package of the active gas. This circumstance involves extended down-time periods in the operation of the microelectronic product manufacturing facility, with resulting adverse economic impact on the facility.
Accordingly, the microelectronic product manufacturing industry has continuing need for improved gas supply sources and monitoring of gas-dispensing operations, for efficient and economic delivery of dilute gases to process equipment.